Gangliosides, such as GM1, GM2 and GM3, are glycosphingolipids (GSLs) comprised of ceramide and at least one acidic sugar. Gangliosides are generally found in the outer leaflet of the plasma membrane (Nojri et al., Proc. Natl. Acad. ScL USA 83:782 (1986)). Gangliosides are involved in cell signaling and act as modulators of receptor activity (Yamashita et al., Proc. Natl. Acad. ScL USA 100(6):3445 (2003)). A number of GSLs are derived from glucosylceramide, which is enzymatically formed from ceramide and UDP-glucose. The formation of glucosylceramide is catalyzed by glucosylceramide synthase.
It has been found that the level of GSLs controls a variety of cell functions, such as growth, differentiation, adhesion between cells or between cells and matrix proteins, binding of microorganisms and viruses to cells, and metastasis of tumor cells. In addition, the glucosylceramide precursor, ceramide, may cause differentiation or inhibition of cell growth and be involved in the functioning of vitamin D3, tumor necrosis factor-α, interleukins, and apoptosis. Sphingols, precursors of ceramide, and products of ceramide catabolism have also been shown to influence many cell systems, possibly by inhibiting protein kinase C.
Defects in GSL metabolizing enzymes can cause serious disorders. For example, Tay-Sachs, Gaucher's, and Fabry's diseases result from enzymatic defects in the GSL degradative pathway and the accumulation of GSL. In particular, GM1 accumulates in the nervous system leading to mental retardation and liver enlargement. In Tay-Sachs, GM2 accumulates in brain tissue leading to mental retardation and blindness. These observations suggest that inhibitors of glycosylceramide synthase can be effective in treating lysosomal diseases such as Tay-Sachs, Gaucher's, and Fabry's diseases. Indeed, glucosylceramide synthase inhibitors have been described for this purpose (see U.S. Pat. Nos. 6,569,889; 6,255,336; 5,916,911; 5,302,609; 6,660,749; 6,610,703; 5,472,969; and 5,525,616).
Recently it has been disclosed that the interruption of the insulin induced signaling cascade may be associated with elevated levels of GM3. It has also been suggested that the cytokine tumor necrosis factor-α (TNF-α), implicated in insulin resistance, results in increased expression of GM3 (Tagami et al., J. Biol. Chem. 277(5):3085 (2002)). Also, it has been disclosed that mutant mice lacking GM3 synthase, and thus lacking in GM3, are protected from insulin resistance caused by a high-fat diet (Yamashita et al., Proc. Natl. Acad. Sc. USA 100:3445-3449 (2003)). These observations suggest that inhibitors of glycosylceramide synthase can be effective in treating diabetes. Indeed, inhibitors of glucosylceramide synthase have been proposed for treating Type 2 diabetes (see WO 2006/053043).
Therefore, agents which inhibit glucosylceramide synthesis, or reduce intracellular content of GSLs, such as GM3, have the potential to treat conditions associated with altered GSL levels and/or GSL precursor levels. There is a need for additional agents which can act as glucosylceramide synthase inhibitors.